1. Field of the Invention
This invention relates to digital wireless communication systems, and more particularly to methods for rapidly assigning traffic channels in digital wireless communications rapidly assigning traffic channels in digital wireless communications systems.
2. Description of Related Art
Wireless communication systems facilitate two-way communication between a plurality of subscriber mobile radio stations or “mobile stations” and a fixed network infrastructure. Typically, the mobile stations communicate with the fixed network infrastructure via a plurality of fixed base stations. Exemplary systems include such mobile cellular telephone systems as Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) systems, and Frequency Division Multiple Access (FDMA) systems. The objective of these digital wireless communication systems is to provide communication channels on demand between the mobile stations and the base stations in order to connect the mobile station users with the fixed network infrastructure (usually a wired-line system).
Mobile stations typically communicate with base stations using a duplexing scheme that allows for the exchange of information in both directions of connection. In CDMA communication systems, transmissions from a base station to a mobile station are referred to as “forward link” transmissions. Transmissions from a mobile station to a base station are referred to as “reverse link” transmissions. The basic radio system parameters and call processing procedures for exemplary prior art CDMA systems is described by the TIA specification which is entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” TIA/EIA/IS-95-A, published in May 1995 by the Telecommunications Industry Association, and referred to hereafter as “IS-95”.
Both voice and data services are available using CDMA communication systems made in accordance with IS-95. However, disadvantageously, data calls use the same airlink protocols, traffic channels, physical layers, signaling methods, call processing schemes and airlink protocols as used by the voice calls. While the prior art call processing schemes and signaling methods are efficient and effective for voice services, they are inefficient for data services, especially when the data services comprise very short duration calls. As described in more detail below, it can take between two and three seconds to establish or “setup” an average voice traffic channel using the prior art call processing schemes. While this setup time may be acceptable for a voice call that, on the average, may have a duration of between 100 and 300 seconds, it is unacceptable for a data call having a duration of only a few seconds, or less. Therefore, an improved technique is needed for assigning data traffic channels in a CDMA communication system. The causes of traffic channel assignment delays in the prior art systems become apparent by reviewing CDMA call flow examples. Therefore typical prior art CDMA call flow examples are now described.
CDMA Call Flow Examples
Table 1 shows a simple call flow example as set forth in IS-95. Table 1 uses the following conventions:                All messages are received without error.        Receipt of messages is not shown (except in the handoff examples).        Acknowledgements are not shown.        Optional authentication procedures are not shown.        Optional private long code transitions are not shown.        
TABLE 1Simple Call Flow Example - Mobile Station OriginationMobile StationBase StationDetects user-initiated callSends Origination Message>Access Channel>Sets up Traffic ChannelBegins sending null TrafficChannel dataSets up Traffic Channel<Paging Channel<Sends Channel AssignmentMessageReceives N5m consecutive validframesBegins sending the TrafficAcquires the Reverse TrafficChannel preambleChannelBegins transmitting null Traffic<Forward Traffic<Sends Base StationChannel dataChannelAcknowledgement OrderBegins processing primary traffic<Forward Traffic<Sends Service Optionin accordance with ServiceChannelResponse OrderOption 1OptionalOptionalSends Origination Continuation>Reverse Traffic>MessageChannelOptionalOptionalApplies ring back in audio path<Forward Traffic<Sends Alert With InformationChannelMessage (ring back tone)OptionalOptionalRemoves ring back from<Forward Traffic<Sends Alert With Informationaudio pathChannelMessage (tones off)(User conversation)(User conversation)
Table 1 shows a simple call flow example wherein a mobile station originates a call. Base station originated calls follow similar procedures. Messages are transmitted from the mobile station to the base station using the access channel. Messages are transmitted from the base station to the mobile station using the paging channel. As shown in Table 1, the mobile station first detects a user-initiated call, and then sends an “origination” message via the CDMA access channel. The access channel is a slotted random access channel. The mobile station transmits on the access channel using a random access procedure. Many parameters of the random access procedure are supplied by the base station in an access parameters message. The entire process of transmitting one message and receiving (or failing to receive) an acknowledgement for that message is called an “access attempt.” Each transmission in the access attempt is called an “access probe.” Within an access attempt, access probes are grouped into access probe sequences. Each access probe sequence comprises a fixed number of access probes. The first access probe of each access probe sequence is transmitted at a specified power level relative to the nominal open loop power level. Each subsequent access probe is transmitted at a power level that is a specified amount higher than the previous access probe.
In normal CDMA operation, when a mobile station user initiates a phone call, the mobile station sends an access probe to the base station. If the access probe is properly received by the base station, the mobile station should receive back an acknowledgement from the base station. Once the acknowledgement is received by the mobile station, the mobile station is instructed by the base station to wait and to stop sending further access probes to the base station. This is necessary because access probes produce interference on the communication channel. The mobile station therefore waits until it is assigned a traffic channel by the base station. The base station then communicates this request for a traffic channel and information about the mobile station to a base station controller (BSC). The BSC performs several administrative functions, possibly including authenticating the mobile station. The BSC then reviews the pool of available resources and allocates an element for the requesting mobile station.
As shown in Table 1, the base station informs the mobile station of the traffic channel assignment by sending a channel assignment message via the paging channel. Once the mobile station receives its channel assignment from the base station, it changes its receive and transmit frequencies, in addition to other relevant parameters, to the assigned traffic channel. The mobile station then attempts to initiate communication on the assigned traffic channel by establishing or “setting up” the traffic channel. if the traffic channel initialization is successful, the mobile station then acquires the traffic channel. The mobile station then begins sending a preamble on the reverse traffic channel to allow the base station to acquire the mobile station. As shown in Table 1, the base station acquires the reverse traffic channel and sends a base station acknowledgement order to the mobile station if the reverse traffic channel was properly acquired. At this point the mobile station and the base station begin negotiating service. The communication link can fail at any point during this negotiation process. However, if the negotiation process is successful, communication commences and a telephone conversation ensues. if the mobile station receives pilots from more than one base station, it may then request the allocation of additional traffic channels from the other base stations.
The prior art traffic channel assignment procedures shown in Table 1 take a relatively long period of time to execute. For example, from the time a base station receives a traffic channel request from a mobile station via the access channel, it typically takes between two and three seconds before a traffic channel is assigned and a base station acknowledgment order is transmitted to the mobile station. As noted above, this service delay is acceptable for voice services wherein the duration of voice calls are typically between 100 and 300 seconds. However, this service delay is unacceptable for data services wherein the duration of data calls are typically only a few seconds, or less. In addition, the assignment of traffic channels utilizes scarce system resources such as specific base station hardware, limited numbers of code channels, and transmission bandwidth (which is required both for tracking and for power control even when no data is transmitted). Therefore, to improve system capacity and throughput, it is advantageous to rapidly de-assign traffic channels whenever the user terminal goes dormant. That is, whenever the user terminal and the base station no longer have information to exchange, it is desirable to rapidly de-assign the traffic channel associated with the mobile station and to quickly re-assign the traffic channel when more data is presented for transmission.
In addition to delaying service to the user (whether the service be voice or data based), delays associated with the assignment of traffic channels create further delays in providing power control of the user terminal (typically a cellular telephone). Because the user terminal's transmission power can vary greatly, it is important to control the power of the user terminal as quickly as possible to avoid unnecessary co-channel interference that can both reduce system capacity and result in loss of the traffic channel. Therefore, it is desirable to both reduce the delays associated with the assignment of traffic channels and supervise the user terminals as quickly as possible. The present invention provides a method and apparatus that address these needs by rapidly assigning traffic channels to mobile stations in a wireless communication system. The present invention also provides a mechanism for rapidly and efficiently controlling the transmission power of the requesting mobile stations.